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Since the rate that a pool size will change in response to a new input or output is a function of the pool’s residence time, please describe how you have determined the residence time of carbon in the different soil pools.

Thank you for the question. It is a great one. I have not tried to measure MRT directly by measuring all inputs and outputs to the system at different stages of forest development. I am currently preparing to density fractionate my soil samples, and it would be great to use 14 C dating to understand the turnover time in those fractions. If you have suggestions of how to do this in a more effective or efficient manner, I would love suggestions. I know this is an important aspect to understand SOM protection in soils and would like to calculate this parameter in a meaningful way for my system.

Thanks again for the question, and I very much would love feedback. I know the IGERT Posterhall is a busy time, but would it be possible to discuss at a later time?

Humans have certainly had a profound impact on soil carbon pools since industrial development and widespread agricultural development. There is an extensive literature on this topic specifically, and some key facts to consider would be the following:

Converting forests to agricultural lands causes soil organic carbon (SOC) pools to decline by 20-50%. This is due to many factors, including erosion, higher microbial respiration in response to different soil temperatures and moistures, destructive tilling, and compaction.

One of the most important factors, however, is the decrease in inputs of carbon to the soil system. By replacing large, woody plants or deeply-rooted grasses with crops, we reduce the aboveground biomass and thus eventually the soil carbon pool.

Generally researchers show that an agricultural field left to naturally regenerate will re-accumulate some of the carbon and nitrogen that had been lost. In some cases, the carbon and nitrogen pools of the soil do recover to match those of places that have never been disturbed.

Thank you for the question and please let me know if you have any more!

I admire your enthusiasm for and strategies in collecting soil data. Can you please explain the significance of the letters above in Figure 1 and clarify the line sets, especially the lower one in Figure 3?

I am sorry I overlooked not including that information in my Figure captions! In Figure 1, the letters represent statistical significance at the p<0.05 level, where different letters indicate that the values are significantly different from each other. I used the Wilcoxon method for multiple comparisons to test significant differences in C and N concentrations and pools between pairs of sites. Error bars represent the standard error of the means.

In Figure 3, I am comparing soil temperatures in a harvested site versus the average of four nearby forested sites. The bars above and below the bottom line are the standard deviations of the hourly temperatures. The temperatures shown in the graph were taken at 55 cm depth. When we installed temperature probes at 3 depths within the soil profile, we were not expecting to see that soil temperature was elevated in the clear-cut forest even at 55 cm depth. This is particularly important when considering that microbial metabolism increases exponentially with temperature. Thus, higher temperatures throughout the soil profile could cause microbes to metabolize stored carbon and release it as CO2.

It does and I really appreciated the additional explanation you added in your answer to re-orient me to the big ideas rather than just assuming I would go back to the poster- especially since this is such a busy time for you too!

1.) Have you run any back-of-the envelope calculations yet to estimate the total potential stock of carbon that could be sequestered in NE U.S. and/or Greenland forests yet? (Fig. 2 seems to suggest you have the data to do this.)

2.) You have presented soil C vs. logging and soil temp. vs. microbial activity as separate issues. Can you tie them together and paint a simple, generalized picture of how these dynamics will influence each other?

3.) Are you using the Greenland component as an interesting contrast to the NE U.S., or are you trying to corroborate the NE numbers? Presumably soil dynamics will be different in Greenland, if for no other reason than temperature will be different?

BTW – I noticed you’re all smiles when running that auger. Have you hit some hard clay or rocks yet? Airborne. . .

Thank you for your thoughtful questions. I will answer them by number, but first, that auger IS very heavy and difficult to move around. This is especially true when trying to hike in to the sampling location. In Greenland, the soils are quite sandy, and we had a relatively easy time drilling to 60 cm depth. In the Northeastern US, however, the process is very slow and laborious and requires hedge trimmers, a pry-bar, and lots of patience when you have to start a new hole. The drill bit can get through some rocks, but locally, it is still a gruesome process.

1) I have not personally measured aboveground C pools at my sites, but starting with the Hubbard Brook experiment in the 1970’s, total forest C pools in this area have been studied in depth. In a mature northern hardwood forest such as the one I work in, it has been estimated that there are 29,062 g C m-2. This number is very large, because it includes everything from mineral and organic soil, dead wood, live trees, tree roots, microbial biomass, etc. Soils here are relatively young, as the Laurentide Glacier retreated from this area only 10,000 years ago, so it is plausible that soil development and carbon accumulation will continue as long as there are no nutrient limitations. As long as Net Ecosystem Productivity is above zero, carbon should accumulate in the ecosystem.

2) Forest harvest, carbon pools and microbial activity can certainly be tied together conceptually. Empirically, I am working on showing this at my sites. Microbial metabolism increases exponentially with temperature according to the Arrhenius Function. This fact suggests that increased temperatures in soils would lead to higher microbial activity, and thus carbon mineralization. One mechanism that may explain the observed differences in carbon pools between harvested and undisturbed forests is the increased microbial respiration with increases in temperature.

3) I will certainly be drawing contrasts between my local work and the work I had the opportunity to do in Greenland. For my thesis, I will compare and contrast my findings about carbon protection in soils. I am working with both sample sets to determine mineralogy, texture, C distribution within aggregates, etc. Interestingly, I am collaborating with a post-doc working on a new global soil C model out of Princeton, and he has found an interesting pattern in the percentage of protected carbon across a latitudinal gradient. Because fresh inputs of organic material don’t decay in the cold arctic, there is more potentially-vulnerable carbon there, whereas in the tropics soil nutrients are cycled very quickly. I am working with him to ground-truth some of his measurements in the mid and high latitudes.

Fun project, important questions, and great study sites! Just wondering though, when you say that your research helps us to decide how to ‘use soils’, I wasn’t clear what you meant? There isn’t any use of soils in Greenland, is there? And while I understand that forest management affects soils, it isn’t direclty using the soil either in the Northeast, or am I missing something? Don’t get me wrong, I certainly understand the need to quantify soil carbon and its contributions to the global carbon cycle. I just didn’t get what one could/would do about it? Enlighten me!

Thank you for your thought-provoking question. Well, the first and most basic answer is that, when we were filming in the middle of the field day, I didn’t articulate my conclusions as well as I should have(: Every time I watch that part of the video I cringe a little bit. Instead of saying my research could help us decide how to ‘use’ soils, I should have mentioned some of these points:
What I hope my research contributes to is an increased understanding of the physical and chemical protection of soil carbon. In relation to the Northeastern US, I hope to strengthen models of soil C cycling and improve our understanding of human disturbances on soil biogeochemistry. Currently, models put forth by the US Forest Service, which are used broadly in economics and forest management, assume no change in soil carbon after forest harvest. In particular, deep mineral soil has been assumed to be unresponsive to disturbances at the soil surface. Because deep soil is not easy to sample, these assumptions about carbon loss and disturbance impacts have largely gone untested. So far, my project is providing some evidence that our aboveground impacts DO actually affect the biogeochemistry below ground.

For Greenland and the Arctic in general, we are are concerned about soil C storage because the release of C from the soil could create a positive feedback to climate change. You are correct that there may not be much we can “do” to slow the melting of permafrost and subsequent mineralization of carbon. However, we know relatively little about what controls C storage or how vulnerable this C actually is. For instance, what size aggregates accumulate the most carbon? In a highly variable, recently-deglaciated landscape, soils of which mineralogy have a greater capacity to bind to and chemically protect soil C? Will the dry soils of the Arctic Tundra support increases in microbial respiration or will microbes be limited by water?

I hope this gets at your question a bit. Hopefully with continued efforts we will have a better understanding of terrestrial feedbacks to climate. Thanks for the opportunity to explain the significance of my research!

Thanks for the comment, Hannah. I am not investigating woody encroachment, however, it is occurring and being studied in the Greenlandic tundra. You could check out the Eric Post lab from Penn State to learn more about vegetative and phenological changes in Greenland. Currently we are using our soil samples from Greenland to understand the physical and chemical protection of carbon within the soil matrix. Basically, how vulnerable is this carbon to mineralization with warming soils and melting permafrost?

Ross- Most of our soils cores made it to 60 cm depth without hitting permafrost. In one location we did make it to permafrost but were unable to extract it after drilling through. We did manage to pull out one piece, and believe it or not it lived up to legend— it was very dark and rick looking and had discreet pieces of organic matter. This was definitely in contrast to the soil above, which was sandy and lighter in color. I can also tell you how it tasted! Once we decided the permafrost chunk had been handled too much to run analyses on, we passed it around for everyone to have a little taste of the 7K-yr-old vegetation. It was gritty and organic.

Hi Ruth, Thanks for the question! We are measuring texture, mineralogy, aggregate size class distribution and the C and N content of different aggregate sizes, in addition to bulk C and N and pH. Basically, we are interested in the physical and chemical properties of the soil that may be influencing C storage.

Very interesting work, Chelsea. In looking at the graphs on your poster, I was interested by the apparent “spike” in the 12-yr-old sites, and then the sort of jumble of responses in the other <100 yr sites. Must be site-specific factors at work here — how have you teased these out to see the signal?

Hello Dr. Drayton. Thanks for picking up on that! The increase in SOM at the 12-yr site was actually predicted based on what we know about the reorganization of carbon in the ecosystem after cutting. Typically a lot of “slash” is left behind after the harvest, and many studies have seen an initial increase in soil C. However, after the C decays and inputs to the system are still low because the forest is not mature, conceptually one could imagine that carbon pools begin to decline. The Hubbard Brook experiment showed that dead woody debris on the forest floor was at a minimum 20 years after harvest, after an initial pulse in the decade after cutting. One hypothesis for differences in carbon pools across the landscape is that C outputs could be increased through the “priming effect,” whereby fresh C added to the system simulates the breakdown of preexisting, ‘stable’ carbon. An increase in mineral soil C at the 12 year site, and then a subsequent decline in C would be consistent with both the priming hypothesis and the dynamics of detrital inputs to the system.

Chelsea, great video! You do an excellent job of explaining the connection between your work in the Northeast and in Greenland. It is also great to see your C storage results. I’m wondering — how recently cleared was the site where you measured temperature? Do you have any similar paired measures of moisture? The 2 degree difference is soil temp is quite remarkable!

The recently-cleared site is ~5 years old. We were also very surprised to see such an increase in temp at 55 cm depth. At the surface this was expected, but not that deep. This supports the idea that microbial activity would be accelerated and could be contributing to larger C outputs.

I do have paired moisture data and just increased the speed of my computer so that I have the RAM to analyze all of it (it’s a LOT of data). During the first summer I took measurements, the differences in moisture were small, however.

This is a great question. I do not have an easy answer, though. This is part of the reason we are looking into the mineralogy, texture, and aggregregate and density classes of the soils in the Greenland area. Without knowing the mechanism by which C is stored, it is difficult to understand the dynamics of the system, especially with steadily-changing climate. One major difference is the fact that the Arctic soils store C in permafrost. That one factor greatly increases Arctic soil’s ability to store carbon, because the carbon that is added to the system does not decay. Moisture must also be considered— the Kanger area is a dry tundra system, while a temperate forest typically receives plenty of moisture for microbes to respire C. Let’s talk more about this if you want. I think it is really interesting to compare and contrast the two systems!

niko horster

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May 22, 2013 | 08:19 p.m.

neat, want to come test carbon sequestration through intensive grazing and keyline plowing in Vershire, Vt. I believe that plowing and commercial ag is one of the worst carbon releases we have, by managing grazing lands differently we can maybe turn this around.

Interesting and important study! Since your work is partly predicated on climate change, it might be important to also consider how past and future changes in soil moisture distribution with depth due to changes in precipitation and ET would affect carbon storage.